As NASA Smacks an Asteroid, Hundreds of Space Rocks Rate as Potentially Risky
Most have almost zero chance of impacting Earth, but that's still higher than zero.
- Finalist for the Nesta Tipping Point prize and a degree in broadcast journalism from the University of Missouri-Columbia.
An illustration of NASA's DART mission.
On Monday evening, NASA will attempt a staple of science fiction in real life for the first time when it tries to redirect an asteroid in space by crashing a spacecraft into it.
The Double Asteroid Redirection Test (or DART) mission spares the Hollywood cosmic cowboy "Let's nuke it!" approach favored in films like 1998's Armageddon, in favor of basically hurling a robot at a space rock from southern California, Matthew Stafford-style. But the target won't be Super Bowl MVP Cooper Kupp. Rather it will be a small asteroid called Dimorphos that poses absolutely no threat to Earth.
See also: Watch Live as NASA Crashes a Spacecraft Into Asteroid Dimorphos
This might make you wonder why NASA spent over $300 million to test its new planetary protection technique and isn't trying it out on an asteroid we're actually worried might hit Earth.
The simple answer is that there are currently no asteroids out there we know of worth losing any sleep over.
The space community uses something called the Torino Scale to communicate the level of certainty and potential damage of a possible future asteroid or comet impact. The scale runs from zero, for an object with almost no chance of impact or that is so small it would do minimal damage, to 10 for an object that is certain to hit us and that "may threaten the future of civilization."
At the moment, of all the thousands of near-Earth objects that have been spotted and cataloged or tracked by astronomers, every single one is scored a zero on the Torino Scale. Every now and then, a new discovery or new observation might rate an object as a one on the scale, which is still not a cause for concern, and so far additional observations or data have always caused the risk score to drop to a zero on the scale.
Since the Torino Scale was adopted in 1999, the highest an object has ever scored was a four, defined as "current calculations give a one percent or greater chance of collision capable of regional devastation." That was for the asteroid Apophis, which initially had about a one in 60 chance of impacting Earth during a predicted close pass in 2029. Astronomers have since ruled out any chance of an impact for the oversized boulder at any point over the next century.
While the risk of any known dangerous object hitting Earth is minuscule and doesn't rank on the scale, that doesn't mean that there is absolutely zero risk.
The European Space Agency maintains a "risk list" of all near-Earth objects that pose a greater than zero risk to the planet. As of Sept. 26, there were 1,416 objects on the list. Every object on the list has a less than 1% chance of impacting Earth. In fact, all but 10 objects have less than a one-10th of 1% chance of impact (that is, a less than one-in-1,000 chance).
Of those top 10 objects to worry about impacting our planet, all are less than 20 meters (66 feet) in diameter, which is small enough that almost all of the asteroid would burn up as it enters our atmosphere, posing essentially no threat to anything on the ground.
There are still some notable objects on the risk list, however. Asteroid 2022KL8 was discovered earlier this year and is set to make a close pass by Earth in the year 2111. With a diameter of 2,000 meters (1.25 miles), it could be a very interesting object for astronomers to study or for future astronauts to even visit.
There's also the next item up on the list chronologically: the car-sized asteroid 2009TB has a one-in-350,000 chance of smacking into the atmosphere this Friday.
Officially, the riskiest asteroid on the list, based upon a combination of its size and likelihood of impact, is asteroid 1979XB, a monster of a rock about a half-mile (700 meters) across with a less than one-in-3 million chance of hitting us in 2056.
If that's as scary as it gets, you begin to see why mission planners didn't bother with aiming DART at anything on the list.
If there's something to be anxious about, it's the objects that we haven't yet discovered. Due to the location of our observatories, humanity has a few particularly worrying blind spots when it comes to picking out objects from the Southern Hemisphere or those coming at us from behind the sun.
The huge bolide that exploded in the atmosphere over Russia in 2013, blowing out thousands of windows in the city of Chelyabinsk below, came from the direction of the sun and went totally unseen by scientists until it was already a fireball blazing above the surface of Earth.
Upcoming missions like NASA's NEO Surveyor are designed to eliminate this blind spot. And hopefully any threatening asteroids it might reveal in the future can be redirected by tackling them with something like DART to redirect them elsewhere.